This invention relates to a method for preparing silver halide emulsions incorporating gallium, selenium, and Group 8 shallow electron trapping dopants, and to the resulting doped emulsions and photographic elements which contain one or more of such doped emulsions.
In the field of photosensitive materials, high sensitivity is generally desired, especially for photographs requiring a high shutter speed and photographs encountering difficulty in obtaining a satisfactory amount of light for exposure. However, highly sensitive photosensitive materials typically have coarse graininess. Silver halide emulsions comprising high percentages of tabular grains have been found to be useful in enabling improved speed/grain performance. Processes for producing tabular silver halide grains and techniques for utilizing the same are disclosed in, for example, U.S. Pat. Nos. 4,434,226, 4,439,520, 4,414,310, 4,433,048, 4,414,306 and 4,459,353. The advantages of the tabular silver halide grains are known in, for example, improving the relationship between sensitivity and graininess inclusive of enhancement of the efficiency of color sensitization by a spectral sensitizing dye. While tabular grain emulsions themselves provide advantageous speed/grain performance, further improvements in the relationship of sensitivity/graininess would be useful.
Various techniques can be used for enhancing the sensitivity of the silver halide emulsion. Research Disclosure, Vol. 176, December 1978, Item 17643, Section I, sub-section A, e.g., states that xe2x80x9csensitizing compounds, such as compounds of copper, thallium, lead, bismuth, cadmium and Group VIII noble metals, can be present during precipitation of silver halidexe2x80x9d emulsions. The quoted passage is followed by citations to demonstrate the general knowledge of the art that metals incorporated as dopants in silver halide grains during precipitation are capable of acting to improve grain sensitivity.
The term xe2x80x9cdopantxe2x80x9d is employed herein to indicate any material within the rock salt face centered cubic crystal lattice structure of the central portion a silver halide grain other than silver ion or halide ion. The term xe2x80x9ccentral portionxe2x80x9d in referring to silver halide grains refers to that portion of the grain structure that is first precipitated accounting for up to 99 percent of total precipitated silver required to form the grains. The term xe2x80x9cdopant bandxe2x80x9d is employed to indicate the portion of the grain formed during the time that dopant was introduced to the grain during precipitation process.
Research Disclosure, Vol. 308, December 1989, Item 308119, Section I, sub-section D, states that xe2x80x9ccompounds of metals such as copper, thallium, lead, mercury, bismuth, zinc, cadmium, rhenium, and Group VIII metals (e.g., iron, ruthenium, rhodium, palladium, osmium, iridium and platinum) can be present during the precipitation of silver halidexe2x80x9d emulsions. The quoted passage is essentially cumulative with Research Disclosure 17643, Section I, sub-section A, except that the metals have been broadened beyond sensitizers to include those that otherwise modify photographic performance when included as dopants during silver halide precipitation. Research Disclosure 308118, I-D proceeds to point out a fundamental change that occurred in the art between the 1978 and 1989 publication dates of these silver halide photography surveys, stating further: The metals introduced during grain nucleation and/or growth can enter the grains as dopants to modify photographic properties, depending on their level and location within the grains. When the metal forms a part of a coordination complex, such as a hexacoordination complex or a tetracoordination complex, the ligands can also be occluded within the grains. Coordination ligands, such as halo, aguo, cyano, cyanate, thiocyanate, nitrosyl, thionitrosyl, oxo, and carbonyl ligands are contemplated and can be relied upon to vary emulsion properties further.
The use of dopants in silver halide grains to modify photographic performance is further generally illustrated, e.g., by Research Disclosure, Item 38957, I. Emulsion grains and their preparation, D. Grain modifying conditions and adjustments, paragraphs (3)-(5). Photographic performance attributes known to be affected by dopants include sensitivity, reciprocity failure, and contrast.
With respect to the metal doping technique, using empirical techniques the art has over the years identified many specific dopants capable of increasing photographic speed. Keevert et al U.S. Pat. No. 4,945,035, e.g., was the first to teach the incorporation of a hexacoordination complex containing a transition metal and cyano ligands as a dopant in high chloride grains to provide increased sensitivity. Marchetti et al. U.S. Pat. No. 4,937,180 teaches the incorporation of hexacoordination complex containing rhenium, ruthenium, osmium, or iridium and cyano ligands in high bromide grains optionally containing iodide. Scientific investigations have gradually established that one general class of speed increasing dopants share the capability of providing shallow electron trapping sites. Olm et al U.S. Pat. No. 5,503,970 and Daubendiek et al U.S. Pat. Nos. 5,494,789 and 5,503,971, here incorporated by reference, as well as Research Disclosure, Vol. 367, Nov. 1994, Item 36736, were the first to set out comprehensive criteria for a dopant to have the capability of providing shallow electron trapping (SET) sites.
There have been several patents or patent applications that have disclosed the practice of introducing Group 13 metals (e.g., Ga or In) during the precipitation of silver halides. The practice of using gallium as a shallow electron trapping (SET) dopant was disclosed in the above referenced patents to Olm et al. and Daubendiek et al., as well as U.S. Pat. No. 6,090,535. These disclosures state that Ga3+as a bare metal ion satisfies the silver halide HOMO and LUMO requirements for an SET dopant. The prior art thus teaches that a broad range of ligands can be used to prepare coordination complexes of gallium as SET dopants for silver halide emulsions. Specific ligands disclosed include halide ligands and strong field ligands that are required for forming octahedral Group 8 transition metal complexes as SET dopants. While [Ga(NCS)6]3xe2x88x92is included among possible SET type dopants in the referenced prior art, the isolation or in-situ preparation of a gallium complex with octahedral coordination has not actually been demonstrated. Instead, where the use of gallium in photographic emulsion has been previously demonstrated, it has been in the form of simple salt such as Ga(NO3)3 (see, e.g., U.S. Pat. No. 5,348,848).
As indicated above, various Group 8 metal complexes have been disclosed as dopants used to enhance or improve photographic properties. These dopants are often used in combination with other dopants introduced during silver growth. U.S. Pat. No. 5,576,172, e.g., discloses use of Group 8 complexes in combination with Ir containing dopants to provide improved speed and low intensity reciprocity. A similar approach is disclosed in U.S. Pat. No. 5,985,508. Group 8 complexes have also been used in combination with Se or Te sensitization processes (U.S. Pat. Nos. 5,609,997 and 5,985,508).
Selenium compounds have also been specifically disclosed for use in doping silver halide emulsions, both alone and in combination with other dopants. U.S. Pat. No. 5,166,045, e.g., discloses the use of Group VIB compounds to form improved photoactive silver halide grains; U.S. Pat. No. 5,164,292 discloses the use of selenium in combination with iridium dopants; U.S. Pat. No. 5,547,830 discloses the use of selenium in combination with iron and/or iridium dopants; U.S. Pat. No. 5,641,619 discloses the use of selenium in combination with iridium and various other dopants. While the use of selenium is known to improve photosensitivity, it would be further advantageous to discover unique dopant combinations which would enable even further photographic performance improvements.
In one aspect this invention is directed towards a process for incorporating dopants in a silver halide emulsion comprising precipitating silver halide emulsion grains in a reaction vessel, wherein at least one gallium dopant, at least one Group 8 metal dopant, and at least one selenium dopant are introduced into the reaction vessel during precipitation of the silver halide grains; where the gallium dopant is introduced in the form of a gallium halide coordination complex of the formula (I):
[RxNHy]3GaX6
wherein R represents a lower alkyl group of from 1-3 carbon atoms; X is Cl, Br, or I; and x is from 1-3, y is from 1-3, and x+y=4; and the Group 8 metal dopant satisfies the formula (II):
[ML6]n
wherein n is xe2x88x922, xe2x88x923 or xe2x88x924; M is a Fe+2, Ru+2, or Os+2 ion; and L6 represents bridging ligands which can be independently selected, provided that at least four of the ligands are anionic ligands, and at least one of the ligands is a cyano ligand or a ligand more electronegative than a cyano ligand. In another embodiment, this invention is directed towards silver halide emulsions formed by such process. In a still further aspect, this invention is directed towards a photographic element comprised of a support, and a silver halide emulsion layer coated on the support comprised of an emulsion obtained by the process of the invention.
As demonstrated in the examples herein, the use of a doped emulsion prepared with a Group 8 metal complex in combination with a selenium dopant and a gallium bromide complex in accordance with the invention has been found to enable the preparation of silver halide emulsions with greater than expected improved speed/grain performance.